Acidization involves treating a formation with an acid (typically hydrochloric, hydrofluoric, and mixtures thereof) in order to dissolve clogging deposits, such as inorganic scales and minerals, to increase formation permeability. It facilitates flow of formation fluids (such as oil or natural gas) into the well from the formation. It also facilitates injection of fluids (such as water, nitrogen, natural gas, or carbon dioxide) through the well into the formation.
Numerous acidization methods have been proposed to cope with different well conditions and special formation problems. In recent years, the increased activity in drilling very deep wells has outpaced the development of suitable high temperature acidization methods. There are three problems common to many prior art acidization methods.
One problem is corrosion of the well equipment, particularly the downhole tubing and casing, which are exposed to the acidizing fluid. This problem is especially serious in acidization of high temperature reservoirs because the reactivities of acids significantly increase at higher temperatures.
A second problem is excessive dissolution of quartz at high temperatures. A sandstone petroleum reservoir can be thought of as a framework of quartz grains, with feldspars, clay minerals, and quartz fragments plugging some of the flow channels between the quartz grains. To improve the permeability of the formation, it is important to dissolve the channel-plugging material without dissolving the quartz grain framework. If too many of the quartz grains are dissolved, the formation can collapse and decrease permeability.
A third problem is precipitation of acid reaction products from the spent acid-treating solution. These precipitates can reduce the formation permeability. Examples of these precipitates include amorphous silica gel and salts of fluosilicic acid.